1. Field of the Invention
This invention relates to power transmission systems, and more particularly to power transmission systems utilizing a plurality of clutches for varying the gear ratio of a transmission by selectively engaging one clutch while substantially simultaneously disengaging another clutch, and even more particularly, this invention relates to the control of such clutches.
2. Description of the Prior Art
Transmissions that shift gears by engaging one clutch while simultaneously disengaging another clutch are known. However, such transmissions, particularly those utilizing multiple plate clutches, have the disadvantage of rough shifts that jolt the vehicle during the shift, particularly if the shift is made under load, such as for example, in a heavily loaded tractor. Since such jolts are quite uncomfortable and can result in damage to the vehicle, or in the case of a tractor, to the farm implement being pulled, various attempts have been made to reduce the aforementioned jolts. Most of these attempts have been directed to the design of the clutches themselves, or to the use of special clutches, such as spray clutches or overrunning clutches that lock up in one direction only; and while such attempts have substantially reduced the jolt produced during a shift, the solution has not been entirely satisfactory since such special clutches tend to be expensive and exhibit poor reliability.